The invention relates to digital echo suppressors for two-way transmission systems, and more particularly to common control digital echo suppressors.
Echos are produced in a transmission system whenever an impedance discontinuity or mismatch exists, such as at the junction between a four-wire transmission channel and a two-wire transmission channel. The annoying effects of echos can be reduced by the use of echo suppressor apparatus which essentially operates to disable the echo return path of a subscriber when he is transmitting. Thus, basically an echo suppressor is a voice-operated switching device which may be situated near one end of a four-wire transmission channel. If echos in both directions are to be suppressed by the device, it is commonly referred to as a full echo suppressor; if echos are suppressed in only one direction, the device is referred to as a split echo suppressor.
In a split echo suppressor, for example, the echo suppressor apparatus nearest a particular subscriber end typically functions to disable the transmit, or outgoing, path from that subscriber when signals from the far-end subscriber appear on the receive, or incoming, path. Thus, echos due to incoming signals on the receive path are prevented from returning to the far-end subscriber over the transmit path. Echo suppressor apparatus at the far subscriber end functions in a similar manner to prevent echos from returning to the near-end subscriber when the near-end subscriber is transmitting. A full echo suppressor essentially combines a pair of split echo suppressors in a single facility located at the near subscriber end, the two split echo suppressors sharing certain of the suppression control circuitry.
During echo suppression in a system utilizing either a full or a split echo suppressor, suppression must be removed from the transmit path when the near-end subscriber breaks in, a condition commonly referred to as double talking since both subscribers are talking at the same time. For this purpose, the echo suppressor includes break-in circuitry for distinguishing between speech signals generated on the transmit path by the near-end subscriber and echo signals on the transmit path due to far-end subscriber speech signals on the receive path. This may be accomplished, for example, by sampling the signals on the transmit and receive paths and comparing the samples, or representations thereof, to determine their relative magnitudes. If the transmit path signals exceed the receive path signals, it is assumed that the near-end subscriber is transmitting and break-in is effected by removing echo suppression from the transmit path. On the other hand, if the transmit path signals do not exceed the receive path signals, it is assumed that the near-end subscriber is not transmitting and the transmit path remains disabled. Similarly, in the case of a full echo suppressor, the break-in circuitry functions to remove suppression from the near-end receive path when the far-end subscriber breaks in.
While the above approach has been generally effective in suppressing echos, it often results in an undesirable amount of clipping of a subscriber's speech signals, particularly if he speaks softly. Consequently, arrangements have been proposed for approximating the echo that may appear on one path due to signals on the other path. The echo signal approximations may then be compared with the actual signals appearing on the one path to determine whether the signals contain speech to be transmitted on the one path. The speed and accuracy with which break-in can be effected without excessive clipping of a subscriber's speech signals, therefore, depends upon the accuracy with which the echo signal approximations are generated.
Further undesirable clipping of a subscriber's speech signals may occur if break-in and suppression are effected substantially instantaneously based upon the comparison of the transmit and receive path signals. It is known to alleviate the problem somewhat by providing a break-in hangover interval such that suppression is not reinserted in the receive path of a subscriber until a predetermined interval of time after the other subscriber stops transmitting. However, prior arrangements have generally required hangover intervals of such length, typically on the order of several hundred milliseconds, so as to permit an undesirable amount of echo to get through. Arrangements providing a suppression hangover interval so as to delay break-in and thus suppression removal until a predetermined interval of time after double talking is detected, on the other hand, may reduce echo but tend to produce unwanted speech clipping.